In the operation of articulate machines, it is common to use a steering mechanism that includes a pair of hydraulic cylinders that are connected between the front and rear frames of the machine on opposing sides of an articulation point. The steering cylinders may be selectively actuated to extend one and retract the other to pivot the front frame with respect to the rear frame to affect a steering maneuver.
Control of the pressurized fluid to the steering cylinders has been accomplished in several different ways. Many steering systems utilized a hand metering unit (HMU) that is connected to the steering wheel and will pump fluid to the steering cylinders in response to rotation of the steering wheel. While this has been known to work effectively, it has been known to cause operator fatigue when operating a machine such as a wheel loader that requires excessive maneuvering.
Attempts to improve upon this system have included the addition of electronic controls that may be incorporated into a steering control other than a steering wheel, such as a handle and lever. This not only reduces the fatigue of the operator, it allows the incorporation of other machine functions, such as transmission controls into the lever. This type of control has been incorporated in the operation of machines such a wheel loaders that are utilized to move material from one location to another, often times by loading the material into an adjacent truck. While this type of control works quite well in truck loading applications, it has been known to exhibit some minor drawbacks when moving material from one site to another utilizing an operation commonly known as "roading".
Other improvements in articulated steering have incorporated the electronic transmission controls into a steering wheel that rotates through a relatively small range, thus allowing the transmission controls to remain incorporated in the steering wheel. The steering wheel is connected to a hydraulic control valve that is mounted on the front frame that articulates with respect to the rear frame upon which the steering wheel is mounted. The connection between the steering wheel and control valve utilizes a pair of U-joints that are positioned in a particular phase with respect to one another to create a mechanical modulation of the control valve as the respective frames articulate. In doing so, the operator is afforded a mechanical feedback as the machine is steered. In addition, the response of the machine is such that initial movements of the steering wheel from its neutral position result in smaller amounts of response at the control valve than what is experienced when the machine is articulated. This has been known to provide a steering system that is much less "jerky" during "roading" operations and is more responsive with less amounts of steering wheel rotation during truck loading operations. While this has been quite successful in the operation of larger wheel loaders, the mechanical modulation is not quite as effective or comfortable for the individual operating smaller wheel loaders that may be operated on highways at higher speeds.
The present invention is directed to overcoming one or more of the problems as set forth above.